I hope those are town MPG readings, because otherwise they'd be horrid. Nice to hear someone did a test and got results for themselves though. :)

 

Well, I used 91 exclusively when I first bought my '85. I then tried 87 for awhile, and didn't notice much difference. When I started running 89, my freeway cruising mpg was slightly higher than both 87 and 91
.

 

I used to only get my 92 octane at Chevron but I started to hear the valves ticking when I accelerated. When I switched to Exxon gas, the tick went away and the Vette runs a whole lot better. Go figure!

 

This was in town. I live about 3 miles from work so I do alot of stop and go driving. I don't really know what is normal because I have only had the car 2 months. In my 4 cyl Toyota Celica I got close to 30mpg under similar driving conditions, I suppose given the much larger L98 this is around normal. :steering:

 

I just had to put 89 in my LT1 today, the station I stopped at was out of 93 :eek: !! Anyway, I did notice that my fuel economy coming into work was a lot better (21.4 vs. 18.6). I was going to ask LT1 owners if they had ever tried 89 and whether it would do anything harmful to the motor. I get just as much pick-up (have 2.59 rear ratio) with the 89 as the 93 and no knocks or pings. Since I drive 70 miles per day, being able to put the 89 in would save a bunch of money! Any thoughts...input?

BTW, ir0nmaiden420, love your handle!! Up the Irons!! (Big Maiden Fan here!! :D )

 

Okay, what about aftermarket chips that call for 91 and up? (yeah, I understand thoughts of the chip being a waste, so let's dismiss that claim for the question's purpose, okay?) :seeya Will that make a difference if 87 or 89 is added?


[Modified by jmrl98, 2:23 PM 3/9/2004]

 

Most chips rely HEAVILY on advancing timing to increase power. Becuause of the advance in timing (plug firing earlier in the combustion stroke) it makes the engine more prone to spark knock, (preignition or pinging). A higher octance fuel can help prevent pinging.

 

You guys kind of get it. As you know gas comes from oil. When it is cracked you get strings of hydrocarbons with different lengths. e.g. propane has 3 atoms... hexane has 6. When gas is referred to as 87 octane (oct meaning it has 8 atoms), it means that it is 87 percent octane. What is the other 13%? That's heptane. Heptane has 7 atoms - and handles compression very poorly before it will spontaneously combusts. Octane handles compression very well and can be compressed a lot before it spontaneously combusts. So the less heptane you have, the more compression it can handle. The reason why you'd make more power on 87 than a higher grade is because 87 burns faster while adding more stable octane causes it to burn slower since it is much more stable. It's pretty straight forward. Basically if your car does not ping, or you aren't getting knock retard, you can go with a lower grade. I haven't tried it though - I just run 91 in the Vette, 89 is only 2% less, and I'd be weary of trying 87 with 10.5:1.

Diesel fuel is very hard to ignite. That's why diesel engines run around 20:1 compression :D

 

Dont different octanes burn differently? Like higher octane burns longer
Can someone affirm that?

 

WickedcC4, did your car make those RWHP and TQ numbers in your sig on 91 octane? Just curious, because for the mods you list, those look like great numbers, plus you're running a custom chip which has to mean your timing is advanced more aggressively than stock.

I have datamaster, and on 93 octane I show no spark retard, although I get some knock counts mostly from startup, which I understand to be normal at startup.

I might try putting in some 91 octane and logging some data w/ datamaster and see if any spark retard shows up. If none then I might try some 89. The quantity of reamining gas in the tank affects the octane rating as the new gas being pumped mixes in, so that is a factor too that may slightly skew results. I've got a tank of 93 now so will probably be awhile before I experiment with 91 octane.


[Modified by Lone Ranger, 4:49 PM 3/9/2004]

 

[QUOTE]
The fuel has nothing to do with torque. Diesels have amazing torque due to the insane cylinder pressure and compression ratios. You need a high octane fuel to hadle those conditions. Torque isnt a function of fuel, its a function of engine design. The fuel simply allows the engine design to do it's work.

Octane doesnt give you more power, putting a higher octane fuel in an engine that doesnt need it doesnt do anything except waste money.
QUOTE]

I don't want to start a flame war or anything, but that's simply not right. Your fuel *IS* your torque/power. An engine without fuel has ZERO power, zero torque, zero horsepower, no matter how it is "designed." See how far it takes you when you run out of gas!

What an engine does is give you a way to tap into the stored energy that is held in your fuel. Good engines can even give you an additional way to tap into the stored power that is in our air (as in, when mixed with fuel).

A good fuel air mixture in a good engine will allow you to tap substantial amounts of energy.

But changing the types of fuel will impact your engine's ability to tap that power simply because different types of fuel store different amounts of tappable energy.

Your low octane fuel, for instance, actually contains MORE power per ounce than does high octane fuel. Low octane fuel is also more volatile. It explodes (i.e. combusts with air) sooner, faster, and easier than does high octane fuel.

Because low octane detonates so easily, you can even run an engine (e.g. diesel) without spark plugs if you want, simply by compressing the fuel/air mixture so much that it explodes on its own.

But for high performance, you want to be able to compress as much fuel/air as possible in order to tap more energy with each explosion in each cylinder.

And to be able to pack more fuel/air into each combustion chamber, we use higher octane fuel in order to reduce the volatility of the mixture. The higher the octane, the more fuel/air we can pack into the chamber. That gives us more overall tappable power simply because we are putting so much more fuel/air in there.

But you've got to remember that all in the world that our engines are doing is extracting power from fuel and air. Our engines don't have torque themselves, but our engines can extract such torque/power from fuel and air.

And different fuels have different amounts of power per ounce.

If you had no compression at all, in fact, your low octane fuel will give you more power than will high octane.

But by packing more high octane fuel into your combustion chamber, your high octane fuel can allow you to extract more power at higher compression ratios than what we can otherwise tap from low octane fuels.

 

'86 w/iron heads is 9.0 to 1 compression. Aluminum heads might have been 9.5. The manual specifies 87 octane.

 

No flame war will be started, I promise. But you have it sort of backwards. Engines are nothing but big air pumps. Fuel powers the pump, and yes fuel does have energy in it, but it only allows the pump to do its job. Engines are not just power taps for fuel. Performance has little to nothing to do with the fuel itself, but more to do with the "air pump" or engine design. While its true fuel does power the air pump, its more the way the fuel is mixed, delivered, and expelled that makes the power. Octane is a rating of a fuels resistance to preignition or burn, not a rating of power per ounce.





[Modified by Chevy Guy, 11:50 PM 3/9/2004]

 

Iroc: yes as far as I know higher octane gas burns longer (slower) than lower octane gas.

Correction on the diesels... :lol: I forgot that diesels inject fuel right when they want it to explode... since diesel fuel has hardly any "octane" rating. The heat of the compression makes it combust. Oops.

Lone Ranger: Yes those numbers were with 91 octane, and without the X-pipe, that is new. As for how far advanced the timing is I'm not sure. I'll look at that whenever I buy my hardware for tuning. The guy that did it, I'm not sure if he advanced it that much. There is probably a good 10 RWHP left in this with modifying the spark tables and whatnot. It would be nice to play with it though before I do the stroker. 91 is the highest you can buy in CA that is readily available. Some '76 stations sell 100 octane for $5 something per gallon... so no thanks. It would be interesting to put lower amounts of octane and do dyno tests. CHP did this one time and they lost HP with more octane... they'd have to bump timing up to get the same amount of power.

Oh and if you have good guess on how much gas you have in your tank just average the numbers and you'll get your octane rating.

 

That's funny!

 

Look who's talking. It was YOUR first post that started the comic relief on this thread.

 

Yes, I agree...unless your the one who typed it. (me<--:smash:) Seeing as how Diesel is injected into the cylinder when they want it to burn, its octane really isnt an issue. Just did a google search and found dieselis actually more like 25 octane........Lesson learned.


[Modified by Chevy Guy, 12:27 AM 3/10/2004]

 

No problem. That's also why diesels often get better gas mileage than comparable engines, simply because their lower octane fuel has more power per ounce than does high octane fuels.

Also jet engines, for instance, burn very low octane kerosene or similar fuels that likewise hold more power per ounce of specific fuel.

 

Nodoubt, I think your signature is long and some of those mods or at least the style in which you list them ... er... are you a reformed ricer? :p:

 

somewhat off the main topic--what would happen if you put diesel in a regular engine by accident, and vice versa?